1. Field of the Invention
This invention relates generally to producing and evaluating parallel digital signals representing a quantity of light received from incident light.
2. Description of the Prior Art
Light grids, also called light curtains, are used whenever the area to be monitored is too wide for a single light gate, or information is needed about the shape, size or position of an object penetrating the monitored area.
This can involve making a dangerous area safe, for instance by monitoring the pivoting range of a robot arm, or determining the actual position of an object, such as a fast-moving tool or machine part.
Light grids with many reception channels are complex and expensive. In the past, serial polling has found wide use. One such “light gate grid” is described in German Patent Disclosure DE32803033A1 (U.S. Pat. No. 5,130,532A). By way of a light source controller and a receiver controller, the light gates are activated in succession. The synchronization can be done, for instance, by way of a connecting line or via synchronization intervals of defined length. Since only one light gate at a time is ever active, mutual influence is practically precluded. By means of the serial evaluation, the digitization can be done by a single evaluation circuit, such as an analog to digital converter or a comparator. However, these advantages are offset by a relatively slow reaction time and only intermittent monitoring. A further disadvantage is that upon switchover of the channels, unwanted interference pulses can occur.
However, if chronologically virtually uninterrupted monitoring or a very fast reaction time is needed, then the only possibility is continuous operation of all the light gates in conjunction with parallel evaluation. In light grids that have 80 or more reception channels, this is quite complicated.
Lighting the area to be monitored with one or more direct-light sources therefore appears advantageous. The transmitter may for instance comprise a chain of light-emitting diodes (LEDs). In conjunction with digitizing the received signals with appropriate resolution, the presence of objects located close to the transmitter can still be demonstrated with certainty.
This necessitates considerable expense for circuitry, which can hardly be managed using discrete components or modules with a small scale of integration. It is not appropriate to use a microcontroller here, since conventional microcontrollers do not have enough ports for parallel processing of such a large number of reception channels. One conceivable solution would be to develop an application-specific integration circuit (ASIC) tailored especially for that task. In many cases, however, this cannot be considered for reasons of cost, because of the comparatively small numbers of items expected to be produced.